1. Field of the Invention
The present invention relates to an electrical device with an electrical line for operation in the examination volume of a nuclear spin tomography device wherein a sheath wave trap tuned to the operating frequency of the nuclear tomography device is installed in the line.
2. Description of the Prior Art
In nuclear spin tomography devices it is occasionally necessary to lead lines to devices which lie within the examination volume of the nuclear spin tomography device. Problems arise therein particularly when lines are located in the region of transmitting antennas. During the transmission phase there are high electromagnetic fields in the examination volume. Coupling events result between the typically shielded lines and the transmitting antenna of the nuclear spin tomography device via the cable shielding by what are known as sheath waves.
The coupling of the antenna field occurs magnetically (inductively) and electrically (capacitively). This coupling can lead to considerable disturbances of not only the image quality but also the electronic components. An even more serious problem is that local field overelevations of the high frequency field can occur due to such couplings. If electrical lines lie too close to a patient, such field overelevations also lead to a sharp increase of the HF power in the neighboring tissue.
Previously, the problem of electrical lines in the transmission field of a nuclear spin tomograph was solved only for passive components. For the present case, this means local coils and sensors such as for EKG, EEG or breathing signals, in particular. In U.S. Pat. No. 5,065,760, primarily two measures were proposed for avoiding the coupling problem. First, the leading of a cable in a shielded cable channel that is arranged in a zone of low HF field strength, and optimally short "free" cable leading to an apparatus arranged at the patient via a plug connector at a patient bed. Second, the installation of a sheath wave filter in the plug connector.
In EKG and EEG sensors, the problem has been further solved in that lines with relatively high electrical resistance are employed which sharply attenuate a HF coupling. For active electrical devices, particularly video displays, sound converters, manually operable control devices and the like, there has been no satisfactory solution until now. Such devices, therefore, usually have not been employed in the transmission field of the nuclear spin tomography device. This can be achieved by transmission with a local coil instead of a whole-body antenna, for example. Such a limitation is unsatisfactory, however, since a more homogenous irradiation of the HF field is achieved with a whole-body antenna than with a local coil.
The known solution for local coils with a sheath wave trap in the plug connector is not, however, sufficient for the use of the abovementioned active electrical devices, since greater line lengths are typically necessary therein and the danger is greater that a cable lies immediately adjacent to the patient. On the other hand, in local coils a lateral connection is typically provided via which a line can be connected to the plug connector without there being a danger that the line lies directly on the patient.
Accordingly, it is an object of the present invention to construct an active electrical device of the abovementioned type such that HF interference is largely avoided when the line lies in the transmission range of a nuclear spin tomography device.